1. Field of the Invention
The present invention relates to communication terminal apparatuses and antenna devices. In particular, the present invention relates to communication terminal apparatuses and antenna devices that can be used in a radio frequency identification (RFID) system.
2. Description of the Related Art
In recent years, an RFID system has been used as an article information management system. In an RFID system, predetermined information is transmitted between a reader/writer that generates an induction field and an RFID tag attached to an article, through non-contact communication based on an electromagnetic field. An RFID tag stores predetermined information and includes a wireless IC (feeding circuit) that processes a predetermined wireless signal and an antenna device that performs transmission/reception of a high-frequency signal. Further, various portable terminal apparatuses that include an antenna device for an RFID system have been recently developed as near field communication (NFC) apparatuses.
Japanese Unexamined Patent Application Publication No. 2001-351083 discloses a non-contact data carrier apparatus that includes a non-contact data carrier, provided with an antenna coil, and an auxiliary antenna for the data carrier. The auxiliary antenna includes a conductive sheet that is arranged outside of a position that is the same as or substantially the same as the innermost circumference of an antenna coil. The area of one side of the conductive sheet is greater than or equal to the area surrounded by the antenna coil. The auxiliary antenna is arranged near the antenna coil of the data carrier. However, in this non-contact data carrier apparatus, the conductive sheet, which functions as a radiation plate, is solid and occupies a large space. Thus, other electronic components need to be mounted in a region other than the region of the conductive sheet, which results in low space efficiency and an increase in the size of the portable terminal apparatus.
Japanese Unexamined Patent Application Publication No. 2007-324865 discloses an antenna circuit that includes an antenna conductor in which a predetermined conductor pattern is provided on the top surface of a base member and a metal surface is arranged on the bottom surface of the base member, where a portion of the metal surface is provided with a cut-out portion that has a shape in which both a starting end and a terminating end of the cut-out portion are open ends in areas in which the metal surface does not exist. However, to obtain necessary communication characteristics, the metal surface and the antenna conductor must be increased in size, leading to an increase in the overall size of the antenna, which is also due to the thickness of the base member. Thus, other electronic components need to be mounted in a region other than the regions of the metal surface and antenna conductor, resulting in low space efficiency and an increase in the size of the portable terminal apparatus.
International Publication No. 2009/142114 discloses a wireless IC device that includes a wireless IC chip which processes a predetermined wireless signal, a feeding circuit substrate which is connected to the wireless IC chip and which includes a feeding circuit including at least one coil pattern, and a radiation plate which radiates a transmission signal supplied from the feeding circuit substrate and which receives a reception signal and supplies the reception signal to the feeding circuit substrate. Further, FIG. 18 of International Publication No. 2009/142114, for example, illustrates a radiation plate defined by a first electrode including an opening and a slit and a radiation plate defined by a second ring electrode that surrounds the first electrode. However, since the second ring electrode completely surrounds the first electrode, the Q factor is likely to be low and there is room to improve the communication characteristics. Further, in International Publication No. 2009/142114, there is no mention of improving space efficiency in relation to other electronic components.